Remote temperature sensing systems are known in the art for the remote detection of overheated regions that may be utilized in, for example, fire detection and suppression systems, etc. One common type of remote temperature sensing system is a linear heat detector. There are a number of different types of linear heat detectors currently available including, for example, digital linear heat detectors and analog linear heat detectors.
Digital linear heat detectors are well-known in the art including, for example, U.S. Pat. No. 2,185,944 entitled FIRE-DETECTING CABLE by Willis Holmes, issued Jan. 2, 1940, the contents of which are hereby incorporated by reference. Generally, a digital linear heat detector comprises a pair of spring conductors made of similar metals. The spring conductors are coated with a special heat sensitive thermoplastic material that melts at a specific temperature. The two conductors are twisted together to maintain a substantially continuous spring pressure between the conductors. Typically, the twisted pair of conductors are wrapped in a protective Mylar® tape, before an outer jacket is extruded over the taped pair.
FIG. 1A is a block diagram of an exemplary digital linear heat detector environment 100A illustrating a typical digital linear heat detector installation. A monitoring circuit 105 is operatively interconnected with a run of a digital linear heat detector 110, which is terminated by a resistor 115. The monitoring circuit 105 maintains a current flow through the digital linear heat detector 110 through the terminating resistor 115, which regulates the current flow through the digital linear heat detector. When current is flowing through the digital linear heat detector at a known level, the monitoring circuit 105 indicates that the system is in a NORMAL state.
FIG. 1B is a block diagram of an exemplary digital linear heat detector environment 100B showing an open circuit 120 caused by a break in the digital linear heat detector. Such a break may be caused by, e.g., physical damage to the linear heat detector. In a situation as shown in environment 100B, the monitoring circuit 105 detects that the current flow has stopped, which causes the monitoring circuit 105 to indicate a TROUBLE state. Typically, the monitoring circuit 105 may sound an alarm or otherwise alert an administrator that the detection capabilities of the system are compromised and that corrective action needs to be taken to restore overheat detection functionality.
FIG. 1C is an exemplary digital linear heat detector environment 100C illustrating operation in the presence of a short 130 that may be caused by a fire or other overheat condition. Illustratively, a fire would raise the temperature higher than the melting point of the special heat sensitive thermoplastic material, thereby causing a short circuit enabling the two conductors to come into contact with each other, which results in an increase in the current through the digital linear heat detector due to the terminating resistor 115 being bypassed. In response, the monitoring circuit 105 will indicate this as an ALARM condition and take appropriate action, e.g., activation of fire suppression systems, etc. However, this leads to a noted disadvantage of digital linear heat detectors, namely, should the digital linear heat detector be physically damaged, thereby causing a short condition, the monitoring circuit 105 will move to an ALARM state with concomitant activation of fire suppression systems. As will be appreciated by one skilled in the art, activation of fire suppression systems in the absence of a fire may result in water damage to a building, goods being stored therein, potential injury to occupants, etc.
Typical digital linear heat detectors 110 have a known resistance, e.g., 0.2 Ohms per foot. Thus, during an ALARM state, the resistance along the digital linear heat detector may be measured to determine the location of the fire.
FIG. 2 is a schematic diagram of a typical cross-section of a digital linear heat detector like that described in United States Publication No. 2010/0142584, published Jun. 10, 2010, by Brian P. Harrington et al., the contents of which are hereby incorporated by reference. The digital linear heat detector 200 comprises an outer jacket 205. The outer jacket 205 is typically an extruded covering that is comprised of some form of polyvinyl. This outer jacket houses two identical inner spring conductors 230 which are coated with a non-conductive heat sensitive material 220, respectively. The coated inner spring conductors are wrapped in a protective tape and/or shield 215, e.g., a Mylar® tape.
Certain recent improvements to linear heat detectors, such as that described in U.S. Pat. No. 7,671,717, issued on Mar. 2, 2010, by Weishe Zhang, et al., the contents of which are hereby incorporated by reference, improve on some of the noted disadvantages of digital linear heat detectors. The Zhang published application details a digital linear heat detector that works to prevent short circuits from causing an ALARM condition. However, a noted disadvantage exists, the Zhang linear heat detector cannot provide positive determination that a thermal event, i.e., an overheat condition, caused the ALARM condition. Furthermore, current systems do not allow for identification of the temperature at the short location.
Currently, the monitoring circuit 105 interprets all forms of short circuits in the same manner, i.e., as an ALARM. This occurs because conventional linear heat detectors cannot distinguish between a short circuit caused by the presence of an overheat condition and a short circuit caused from physical damage to the line (e.g., kinks in the line, animal damage, etc.). Without some form of temperature identification process, mechanical shorts/physical damage may result in spurious activation of fire suppression systems.